Positive displacement hydraulic pumps often operate at variable speeds especially when the pump is in a vehicle power steering system or a vehicle automatic transmission. The pump is driven by the vehicle engine and therefore must operate through the entire engine speed range. The speed at which the pump is driven can exceed 6000 rpm. When the pump speed is high, cavitation may occur at the pump inlet.
In power steering systems such as that shown in U.S. Pat. No. 5,802,848, it is known to position a nozzle in line with a venturi tube to improve the inlet pressure condition of the pump. The power steering system, however, does not employ a filter at the pump inlet. Automatic transmissions, on the other hand, do position a filter element in serial flow relation upstream of the pump inlet. The filter element is known to provide a flow restriction. The sump is typically at atmospheric pressure. Therefore, the pressure available to fill the pump chambers is typically atmospheric pressure minus the pressure drop across the filter.